1. Field of the Invention
The present invention relates to a substrate for large-sized photomask for use in pattern transfer in lithography for the production of display devices.
2. Description of the Related Art
A photomask is known as a transparent substrate comprising an opaque layer pattern formed thereon. As a substrate for photomask there is used a substrate having a rectangular surface made of synthetic quartz glass or the like. The surface of such a substrate is normally mirror-polished taking into account light scattering during the use of photomask. A technique involving the mirror polishing of the side portion (an end surface and a chamfered surface) of a substrate for the purpose of preventing adverse effects on quality such as foreign matter defects caused by attachment of dust present in abraded grooves left in the side portion of the substrate to the surface of the photomask during its production is disclosed in Japanese Patent Unexamined Publication JP-A-56-46227.
Examples of photomasks include photomask (reticle) for use in pattern transfer using a reduction projection exposure apparatus (step-and-repeat scanning system or reduction projection aligner) (stepper) during the production of semiconductors and photomask for use in pattern transfer using a mask aligner which is an exposure device employing an equi-magnitude exposure system during the production of display devices such as TFT array for liquid crystal display. The photomask for stepper and the photomask for mask aligner have different sizes. The photomask for stepper is normally in the form of square having a side of 5 or 6 inch. The photomask for mask aligner is normally in the form of rectangle according to the screen size of monitor and TV display. With the recent demand for the increase of the area of display screen and the production of a large number of screens at a time, the trend is for more substrate for photomasks for this kind of photomask to be designed larger.
Referring to photomask for the production of semiconductors, it has been actually practiced to mirror-polish the side portion of substrates as described in Japanese Patent Unexamined Publication JP-A-56-46227. However, the large-sized photomask for display devices are not required to have a mirror surface on the side portion thereof. Thus, it is the actual fact that photomask for display devices are still available having a roughened side portion. In some cases, photomask for mask aligner is detected on the side portion of the substrate. In this case, if the side portion of the substrate has a mirror surface, reflected light cannot be detected. Thus, it has been desired to roughen the side portion of the substrate.
However, with the recent progress of the reduction of size of elements of pattern and the enhancement of precision of pattern in the production of photomask for display devices, it has been necessary that foreign matter defects attached to photomask during the production procedure be more severely controlled.
It has heretofore been practiced to abrade the end surface (surface T) and the chamfered surface (surface C) by rotating a diamond tool having a proper roughness as shown in FIG. 2. However, the surface thus finished has abraded grooves formed thereon in the direction of abrasion. Particles 2 are potentially stored in these abraded grooves 1. These particles 2 are gradually discharged from the abraded grooves 1 when the substrate is cleaned. Then, the particles 2′ are attached to the surface of the photomask. Under these conditions, a photomask satisfying the recent demand for defect can no longer be obtained even after many times of cleaning (see FIG. 3).
Further, most retainers such as photomask storage tool and cleaning device are adapted to support the substrate at the side portion thereof. Thus, a problem arises that when the rough side portion of the substrate comes in contact with the aforementioned tool, the tool is scraped to generate dust.
Thus, it can be proposed that the aforementioned large-sized photomask, too, has a mirror surface on the side portion of the substrate. In general, however, photomasks are often handled by hands on the side portion of the substrate during its production and use. With the recent trend toward the increase of the size of photomasks, the substrates therefore, too, have been designed to have a greater weight (about 1 kg to 15 kg). Thus, special care must be taken in handing of these photomasks. Under these circumstances, when the side portion of the substrate has a mirror surface, it is very slippery, making it more likely that the photomasks can drop when handled. Thus, a great problem arises that these photomasks cannot be safely handled.